Salinity anomaly as a trigger for ENSO events

According to the classical theories of El Niņo and the Southern Oscillation (ENSO), subsurface anomalies in ocean thermal structure related to thermocline displacements are precursors for ENSO events and their initial specification are essential for skillful ENSO forecast. It has also been noted that ocean salinity in the tropical Pacific (particularly in the western Pacific warm pool) can vary in response to El Niņo events, but its effect on ENSO evolution and forecasts of ENSO has been less explored. Here we present evidence using a state-of-the-art prediction system that, in addition to the passive response, salinity variability may also play an active role in ENSO evolution, and thus important in forecasting El Niņo events. By comparing two forecast experiments in which the interannually variability of salinity in the ocean initial states is respectively included and excluded, the salinity variability is shown to be essential to correctly forecast the evolution of the 2007/08 La Niņa starting from April. With realistic salinity initial states, the tendency to decay of the subsurface cold condition during the spring and early summer 2007 was interrupted by the positive salinity anomalies in the upper central Pacific, which works together with the Bjerknes positive feedback and contributes to the development of the La Niņa event. Our study suggests that ENSO forecasts will benefit from better observations of salinity.